Beran et al., U.S. Pat. No. 4,508,842, patented Apr. 2, 1985, describe an ethylene polymerization catalyst comprising a supported precursor of vanadium trihalide/electron donor complex and alkylaluminum or boron halides, when combined with alkylaluminum cocatalyst and alkyl halide promoter, (hereinafter in this specification and in the claims is called "Beran et al. catalyst system") provides enhanced polymerization and productivity plus a superior polyethylene product. Beran et al. describe a process and a catalyst system for producing polyethylene that involves polymerizing ethylene with or without at least one C.sub.3 to C.sub.10 alpha-olefin monomer in the gas phase at a temperature between about 30.degree. C. to about 115.degree. C. wherein the monomers are contacted with a catalyst composition comprising supported precursor vanadium compounds and modifiers which are impregnated on a solid, inert carrier. The catalysts utilized by Beran et al. differentiate in comprising a supported precursor, a cocatalyst and a promoter in which the supported precursor is a vanadium compound and modifier impregnated on a solid, inert carrier. The vanadium compound in the precursor is the reaction product of a vanadium trihalide and an electron donor. The halogen in the vanadium trihalide is chlorine, bromine or iodine, or mixtures thereof. A particularly preferred vanadium trihalide is vanadium trichloride, VCl.sub.3. The electron donor is a liquid, organic Lewis base in which the vanadium trihalide is soluble. The electron donor is selected from the group consisting of alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic ketones, aliphatic amines, aliphatic alcohols, alkyl and cycloalkyl ethers, and mixtures thereof. Preferred electron donors are alkyl and cycloalkyl ethers, including particularly tetrahydrofuran (THF). Between about 1 to about 20, preferably between about 1 to about 10, and most preferably about 3 moles of the electron donor are complexed with each mole of vanadium used.
The promoter, according to Beran et al., has the formula: EQU R'.sub.b CX'.sub.(4-b) (I)
wherein R' is hydrogen or unsubstituted or halosubstituted lower alkyl, i.e., up to about C.sub.6 containing alkyl; X' is halogen; and b is 0, 1 or 2. Preferred promoters include flouro, chloro or bromo substituted ethane or methane having at least 2 halogens attached to a carbon atom. Preferred promoters include CCl.sub.4, CHCl.sub.3, CH.sub.2 Cl.sub.2, CBr.sub.4, CFCl.sub.3, CH.sub.3 CCl.sub.3, and CF.sub.2 ClCCl.sub.3. Particularly preferred promoters are methylene dichloride (CH.sub.2 Cl.sub.2), 1,1,1, trichloroethane, (CH.sub.3 CCl.sub.3), and chloroform (CHCl.sub.3). Between about 0.1 to about 10, and preferably between about 0.2 to about 2, moles of promoter are used per mole of cocatalyst.
The Beran et al. catalyst system is produced by first preparing the supported precursor. The vanadium compound is impregnated onto the carrier. The modifier, dissolved in an inert solvent, such as a hydrocarbon, is then mixed with the vanadium impregnated carrier. The cocatalyst and promoter are added to the supported precursor either before and/or during the polymerization reaction. The cocatalyst and promoter are added either together or separately, and either simultaneously or sequentially during polymerization. The cocatalyst and promoter are preferably added separately as solutions in inert solvent, such as isopentane, during polymerization.
Thus the promoter is a separately fed component to the polymerization reaction. There are a number of disadvantages to the separate feeding of the promoter to the polymerization reaction. One obvious disadvantage is the additional step required of feeding the promoter to the polymerization reaction with its attendant problems of inaccurate feed, mixing problems, contact with the other catalyst components, and the like. A particularly vexing problem with the separate feed of the promoter to the reaction is associated with the volatility of the halohydrocarbon precursors.